The invention relates to a headbox for a paper machine or the like.
On a headbox known through DE-AS 1 135 744 (corresponding to U.S. Pat. No. 2,928,464), each flow guide wall is equipped with a means for adjusting the basis weight cross direction (CD) profile. This means is constructed in the form of a thin plate which limits the flow guide wall against the stock channel and extends freely beyond a supporting rib up to the stock discharge opening. Viewed in the direction of stock flow, the plate is supported on its rear side shortly ahead of the supporting rib on a plurality of bellows-shaped pressure chambers. These are arranged in close succession across the width of the machine and can be altered singly in their thickness by a pressure fluid. In this way it should be possible to reduce the effective width of the stock discharge opening by controlled lifting of the opening-side plate section from the flow guide wall. This known embodiment, however, is disadvantageous in so far as an increase in the thickness of one or more pressure chambers first of all causes the opening-side end section of the plate, upon which the stock stream impinges, to swivel about the rib, which rib acts as a swivel axis. This swivel movement causes, instead of the desired reduction of the effective width of the stock discharge opening, first of all the opposite effect: a widening of the opening. Only when the opening-side end section of the plate lifts from the rib is a reduction of the effective width of the stock discharge opening caused. There is then the risk that the end section of the plate unsupported in the direction of stock flow beyond the pressure chambers will be excited to vibration, which at least causes disturbances in the basis weight machine direction (MD) profile of the web.
DE-OS 25 29 768 (corresponding to U.S. Pat. No. 4,008,123) relates to a headbox with a machine-wide, movable flow guide wall which bears a profile bar for adjustment of the effective width of the stock discharge opening. Engaging at the two ends of the flow guide wall are force members constructed in the form of pneumatic bellows. These are supported on rigidly shaped parts of the headbox structure. The force members are intended to compensate for heat and load deflection of the flow guide wall. A correction of the deflection line of the flow guide wall achieved by the force members also acts on the profile bar. Inherently, this is undesired, as the adjustment of the profile bar orientates itself not on the deflection line of the flow guide wall but on the distance (width of opening) to the opposite fixed flow guide wall. The force members engaging the end faces of the movable flow guide wall would be completely insufficient for a correction of the profile bar adjustment because the high demands made on accuracy of the width of the stock discharge opening require an adjustability in small distances across the machine width.
DE-PS 29 42 966 (corresponding to U.S. Pat. No. 4,326,916) shows a headbox nozzle with a movable flow guide wall on whose opening-side end face a profile bar slidable approximately at right angles to the stock discharge opening is supported. In a machine-wide support member engaging the profile bar is inserted a pressurizable tube. This is supported on the side of the profile bar facing away from the end face of the flow guide wall. The purpose of this tube is to press the profile bar in respect of its load from the impinging stock flow against the movable flow guide wall. The tube can be disassembled into several sections to be able to exert a higher loading pressure at some points of the web width on the profile bar than in the other areas of the web width. An adjustment of the profile bar to set the width of the stock discharge opening is not the purpose of this known embodiment and also not possible because the direction of effect of the loading force exerted by the tube is at right angles to the direction of movement of the profile bar.